Optical communication systems are usually based on high purity silica optical fiber as the transmission medium. Conventional terrestrial systems are typically designed to transmit optical signals in a wavelength range where longer wavelength components are subject to slightly longer propagation time delay than shorter wavelengths (positive chromatic dispersion). To prevent this dispersion from deteriorating the information content of the optical signals, early systems used a single channel at a wavelength where dispersion is low or zero.
As it has become desirable to utilize many channels over a wider range of optical wavelengths (WDM systems), chromatic dispersion has required more precise compensation. WDM systems are important for their ability to transmit vast amounts of information and for their ability to incorporate network functions such as add/drop and cross connecting. But as the bit rate of WDM channels increases, chromatic dispersion compensation becomes critical.
Typical dispersion compensation schemes for WDM systems involve the use of dispersion compensating fiber having negative dispersion characteristics. The transmission fibers used in terrestrial systems typically exhibit net positive chromatic dispersion which, for WDM systems, cannot be precisely compensated by dispersion compensating fiber. Although segments of such fiber can be used to compensate the accumulated dispersion in a transmission fiber span, optimum compensation is usually achieved only for chosen channels (typically in the middle of the transmission band). There remains a residual wavelength dependent dispersion in channels located at the extremes of the transmission band due to the dispersion slope.
Compensating the accumulated dispersion of the extreme channels can require a dispersion compensating grating (DCG). DCGs are chirped fiber Bragg gratings used in reflection mode and oriented so that the long wavelengths are reflected first before short wavelengths. In this manner, optical pulses broadened due to the accumulated positive chromatic dispersion can be recompressed in time. Typical arrangement using DCGs are described in U.S. Pat. No. 4,953,939 issued to R. E. Epworth on Sep. 4, 1990 and U.S. Pat. No. 5,701,188 issued to M. Shigematsu et al. on Dec. 23, 1997, both of which are incorporated herein by reference. One of the main advantages of using DCGs is that the amount of dispersion and the dispersion slope can be easily adjusted by setting the grating chirp parameters. Another advantage is their low non-linearity compared to conventional dispersion compensating fibers.
A major drawback of typical DCG compensation is cladding loss of short wavelength channels. Cladding loss in a Bragg grating is due to coupling of the forward-propagating core mode to backward propagating cladding modes. It only occurs on the shorter wavelength side of the Bragg reflection peak because of a phase matching requirement. In dense WDM systems using DCGs, the short wavelength channels can experience cladding loss of several decibels. Accordingly, there is a need for a WDM system having reduced short wavelength loss.